1. Technical Field
This invention relates to post-sintering densification of powder metal preforms such as gears and the like.
2. Related Prior Art
Powder metal gears and other powder metal components have long offered a low-cost alternative for wrought components. Powder metal forming eliminates the high cost associated with machining. However, traditionally prepared powder metal components have not exhibited sufficient strength and dimensional tolerances necessary to serve as low-cost alternatives to applications requiring high strength, such as transmission gears.
Increasing the density of the compacted and sintered of a powder metal component can increase its strength. One technique previously known to increase the density of sintered powder metal components is to roll form the components. For example, a powder metal gear can be rolled between forming dies to increase the density of the component, especially at the surface, as shown in U.S. Pat. No. 5,711,187. One disadvantage of roll forming is that the density of the powder metal component may not be uniform after the roll forming process. This problem can be somewhat alleviated by reverse roll forming; however, the additional processing steps associated with reverse roll forming add cost and complexity to the process and may not eliminate the problem.
Powder metal components can also be subjected to impact forging to increase density. U.S. Pat. Nos. 2,542,912 and 5,009,842 describe impact forging in greater detail. During impact forging, the powder metal component is urged through an opening of a forming die which shapes the component. The forming die has a closed end and the component is pressed against the closed end to enhance or complete densification. The densified component is removed from the open end of the forming die. Impact forging is generally not amenable to high volume manufacturing.